Isentropic equation of state of 2-flavor QCD

Using Taylor expansions of the pressure obtained previously in studies of 2-flavor QCD at nonzero chemical potential we calculate expansion coefficients for the energy and entropy densities up to $\mathcal{O}({\ensuremath{\mu}}_{q}^{6})$ in the quark chemical potential. We use these series in ${\ensuremath{\mu}}_{q}/T$ to determine lines of constant entropy per baryon number ($S/{N}_{B}$) that characterize the expansion of dense matter created in heavy ion collisions. In the high temperature regime these lines are found to be well approximated by lines of constant ${\ensuremath{\mu}}_{q}/T$. In the low temperature phase, however, the quark chemical potential is found to increase with decreasing temperature. This is in accordance with resonance gas model calculations. Along the lines of constant $S/{N}_{B}$ we calculate the energy density and pressure. Within the accuracy of our present analysis we find that the ratio $p/ϵ$ for $Tg{T}_{0}$ as well as the softest point of the equation of state, $(p/ϵ{)}_{\mathrm{min}}\ensuremath{\simeq}0.075$, show no significant dependence on $S/{N}_{B}$.